TechArchive

Original article date: July 2000

Analogue or digital? Founder and MD of Status Instruments BRIAN TURNER firmly belives that it will take another 20 years before fieldbus becomes a realistic standard for devices such as temperature transmitters

Although at first glance it may appear that in-head temperature transmitters are basic, commodity products whose development is relatively static, nothing could be further from the truth. Moves to reduce size and cost, whilst improving performance and flexibility, have had a major impact over the last few years, with emphasis on low cost of ownership product.

The advent of fieldbus has sparked both advancement and controversy in control instrumentation circles but, despite the appearance of the first fieldbus compatible devices, this technology has, so far, had little or no impact on the temperature transmitter market. In the meantime, it is likely that the low cost, simple pushbutton devices for the low end applications and HART compatible digital transmitters for the more demanding applications, will enjoy continued growth and development. Perhaps the proprietary ’smart’ devices will become squeezed into a middle ground.

Temperature transmitters fall broadly into analogue and digital categories. Both types offer their own specific advantages, primarily in terms of cost versus performance and features. Analogue transmitters are still widely used in many industries and offer simple, reliable technology at low cost. Early versions were bulky and were hand-built using conventional components. Advances in surface mount technology and new component designs have allowed engineers to reduce the overall size allowing the transmitters to be manufactured to the same dimensions as a connecting block, thus enabling them to fit within a standard temperature probe housing.

This envelope has, in the majority of cases, remained unchanged and become the industry standard. But standard analogue devices are limited in their flexibility. Typically, they are made to order or require individual set-up via solder links and potentiometers. What’s more, units are specifically designed for particular temperature sensors only. These shortcomings increase stock holding and spare part inventory for distributors and users, as well as increasing delivery times.

Most manufacturers have tended to neglect analogue transmitter development in recent years, preferring to focus their efforts on digital technologies. Status, however, has combined the performance of a digital device with the simplicity of re-ranging resulting in a new low cost versatile transmitter. The new 203P model uses surface mount technology and has a pushbutton set-up feature. Instead of having to manually solder links inside the unit to select the temperature range and then adjust potentiometers, calibration is done simply and swiftly via an internal push button switch. The built-in LED indicates correct set-up. A thermocouple version achieves full galvanic isolation through the use of the internal transformer instead of relatively costly opto-isolators. Unusually, the 203P is a sub-(UK pounds)50 product.

Even so, modern digital ’smart’ transmitters offer distinct advantages for both suppliers and users in terms of performance and product flexibility. One basic unit can be configured very quickly, via PC, for different sensor types, operating range and damping parameters – factors which significantly reduce stock holding requirements and virtually ensure ex-stock delivery. However, the increased sophistication in smart devices has significantly increased the electronic component count – a factor which places severe challenges on manufacturers.

Manufacturers have developed remarkable packaging skills to keep new designs within the standard in-head space envelope. In parallel, the designs must also meet the stringent EMC requirements for both immunity and emissions. Despite these restrictions, Status has still found the room to incorporate physical barriers for the all important input-output isolation. Although some transmitter manufacturers rely on the probe itself to provide the isolation, Status is a firm advocate of integral isolation barriers within the transmitter, ensuring that none of its designs can malfunction through ground loop errors. The standard plastic enclosure is still retained, with the products still meeting the most stringent EMC requirements.

Temperature transmitters have retained their 4-20mA output across the range of designs. This faithful and reliable output has withstood the test of time throughout dynamic development changes and with digital devices it is now possible to define the failure mode as either ‘hi’ (upscale) or ‘lo’ (downscale) status.

Use of fully HART-compatible devices has increased over the past decade. HART is a communications protocol used to ‘talk’ to field devices digitally whilst, at the same time, using the industry standard 4-20mA analogue transmission. It is widely used in process industries, such as chemical, oil refining, pulp and paper, food and pharmaceuticals.

Versions of both analogue and digital devices are available for use in hazardous areas. Due to the low power characteristics of transmitters, protection is provided by way of intrinsic safety, with products meeting both CENELEC and FM (Factory Mutual) requirements.

• Status Instruments

July 2000